Size of cargo/shipping containers (herein after sometimes referred to as “containers”) is often expressed by a twenty-foot equivalent unit (TEU) term. The TEU is an inexact unit of cargo capacity often used to describe the capacity of container ships and container terminals. One TEU container is generally 20′ (6.1 m)×8′ (2.4 m)×8.5 feet (2.6 m) (length, L×width, W×height, H) in size. A TEU is based on the volume of a 20-foot long intermodal container, a standard-sized metal box which can be easily transferred between different modes of transportation, such as ships, trains and trucks. While the TEU is not itself a measure of mass, some conclusions can be drawn about the maximum mass that a TEU can represent. The maximum gross mass for a 20-foot (6.1 m) dry cargo container is 24,000 kilograms (53,000 lb). Subtracting the tare mass of the container itself, the maximum amount of cargo per TEU is reduced to approximately 21,600 kilograms (48,000 lb).
Twenty-foot, “heavy tested” containers are available for heavy goods such as heavy machinery. These containers allow a maximum weight of 67,200 pounds (30,500 kg), an empty weight of 5,290 pounds (2,400 kg), and a net load of 61,910 pounds (28,080 kg).
A two TEU has an L×W dimension of 40′ (12.2 m)×8′ (2.4 m) size which is also sometimes referred to as forty-foot equivalent unit (FEU). Regardless of the size of containers, each is generally limited to have a payload of not much more than 30 tons. These containers are designed and manufactured according to IICL standards for loading and sealing intact onto container ships, railroad cars, planes, and trucks. The containers are constructed with sturdy metal (usually aluminum or steel based) and structured to withstand rough handling. The containers (loaded or emptied) are often stacked on top of another for storing and warehousing.
Containers can be owned by a shipper, a leasing lines, or a carrier, for example. When the goods are unloaded from the containers, the containers are stored for a future shipment. Common container depots require proper setup to manage and handle container transfers and stacking. This involves a lot of manual intervention. The container depot also requires repair and maintenance facilities for handling, welding, cutting, spray jet cleaning, painting and metal treatment for the containers.
The containers are either stored at port or with common container depot who handles containers from different owners. The containers require regular inspections, testing, proper storage and handling for reuse. Storage and handling of these containers are often a concern.
Containers are normally stored in open spaces. Stackable height of containers is limited by the equipments used. Current container depots may be stacked up to 10 containers high. A 10 hectares area may accommodate 10,000 one TEU containers with about 10 containers in a stack. Due to the large size of the container stackers, the numbers of container stackers operating, and access ways for tow heads and trailers in the yard, the container storage area usually occupies only about 60% of a total land area of the container depot. The remaining 40% of the total land area is left for operational use.
A container stacker operator's cabin is usually at the same height as the 3rd or 4th stack of containers stored. The operator has to look up and observe carefully while stacking the containers above this level to ensure a proper stacking. The higher the stack height, the more difficult it would be to observe as the container stacking may be out of the operator's line of vision. Improper handling during lifting may cause a container to fall resulting in serious injuries or fatality.
Container loading and unloading operations include survey, inspection and verification of the identification of containers. This is time consuming. Many container depots carry out their surveying on public roads which results in heavy traffic congestions. It is common to see up to 20 or more container trucks along the road outside the loading/unloading container depot. Such traffic congestions not only block out the road and cause traffic jams, it may potentially give rise to road safety issues.
For container depots, therefore there is an existing need to enhance land plot ratio and reduce depot footprint, and to address operational safety and traffic congestion. In a region where land is scarce and expensive, such as Singapore, Hong Kong, Shanghai etc., maintenance of a high level of safety, security and optimal storage is always a challenge.